First time upgrading RAM- what should I do?

I have 2GB of RAM currently on my machine, and since taking the advice from another member up here, I installed the x64 version on Win 7. Anyway, so I have 4GB of memory coming in tomorrow. The RAM I have installed now is ~800 mhz speeds I think. The ram I have coming in is 1333 mhz, which is the most my board can handle. I've read where if you install different speed memory sticks, it will automatically downgrade and run at the lowest setting of all the memory sticks. So, my question is, would it be better just to take out the 2Gigs I have in now, and just run the 4gigs I'm getting in the mail? OR would it be best to just add to it, and have 6 total? Some friendly advice would be much appreciated. Thanks.

I would run with just the 4GB. You could try with the other 2GB inserted as well but, as you state, if you do this you will only be running at the speed of the slowest RAM. In addition, you may have memory timing issues if the CAS and other parameters of the modules are different.

It's always best to run match ram. If you run mismatched ram you have a chance of all kinds of problems. You might get away with it but why take the chance.

In order to install the $GB of RAM, you may have to remove the 2GB of RAM you have now, depending on how your RAM is configured. Do you have 4 RAM slots? I have 4 slots, with 2 slots having matching 800mhz DDR2 (2GB ea) and 2 slots of 800Mhz DDR2 (1GB ea) for a total of 6GB and it's running very well. Try it both ways and compare notes to see what is best for your system. Good luck!

Go to the Crucial memory web site, use the Crucial Memory Advisor Tool or enter your PC model or motherboard model if its a custom built system and Crucial will list how much memory your system can take and the compatible memory (If you have a Dell PC Crucial is recommended as Dell's have a history of memory compatiblity issues with non-Dell memory, Crucial is guaranteed to work in Dells).
www.crucial.com

If you have been using just 2GB up to now then i take it you dont have high RAM usage applications currently so upgrading to just 4GB would be suffice, and the extra frequency you will be getting by removing your old sticks would be a little bonus for speed depending on your other HW